Ignition of Thermite Using the Potassium Chlorate “Rocket” Reaction: a Systematic Demonstration of Reaction Chemistry Brett A

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Ignition of Thermite Using the Potassium Chlorate “Rocket” Reaction: A Systematic Demonstration of Reaction Chemistry Brett A. McGuire,*,†,‡ P. Brandon Carroll,‡,§ Adam N. Boynton,‡,§ Jeﬀrey M. Mendez,‡ and Geoﬀrey A. Blake‡,∥

† National Radio Astronomy Observatory, Charlottesville, Virginia 22903, United States ‡ Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125 ∥ Divison of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, United States

ABSTRACT: Presented here is a set of demonstrations that are used as visual tools for engaging students in a discussion of reaction chemistry and thermodynamics. Students are fi rst shown a series of simple exothermic reactions: (1) the reaction of H2SO4 with sugar, (2) the decomposition of KClO3, and (3) the reaction of H2SO4 with KClO3. These three basic reactions are then combined as a pyrotechnic chlorate demonstration in which fi H2SO4 is used to ignite a KClO3 + sugar mixture. It is nally shown that this latter reaction is very effective for igniting the highly exothermic thermite redox reaction.

KEYWORDS: First Year Undergraduate/General, High School/Introductory Chemistry, Demonstrations, Physical Chemistry, Hands-On Learning/Manipulatives, Reactions, Kinetics, Thermodynamics, Oxidation/Reduction

■ INTRODUCTION demonstration, students are shown how the individual The use of the thermite reaction as an exciting demonstration constituent reactions of (1) H2SO4 with sugar, (2) H2SO4 with KClO producing HClO , followed by the combustion of for young science students is documented in this Journal as 3 3 sugar, and (3) H SO with a KClO −sugar mixture can be early as 1930. In his article, J. E. Nelson describes the melting of 2 4 3 combined to produce the exciting “chlorate rocket” pyrotechnic a steel nail with thermite, tying the chemistry concept of display. This reaction is then used to ignite the thermite in a displacement reactions to the real-world application of demonstration of redox chemistry and reaction barriers. industrial welding.1 Since then, the refinement of the thermite Here we outline the procedures for each reaction, as well as reaction demonstration has been documented several times. In the chemical principles to be demonstrated at each step. particular, the ignition of the reaction, which possesses a large activation energy, has been a key topic of discussion. ■ DEMONSTRATIONS In 1937, O. C. Klein documented the use of potassium 2 For best effect, demonstrations should be performed in order, permanganate (KMnO4) as an ignition system. This system remained unchanged in the public view until 1952, when C. P. with discussion of the chemistry before and after each − demonstration. Brockett demonstrated that the addition of a BaO2 aluminum powder mixture greatly improves the success rate of the H2SO4 + Sugar 3 ignition. A secondary method for ignition was proposed by The procedure is well-described in the literature.7 70 g of Fortman and Schreier in 1993 involving a reactive mixture of common granulated sugar (C12H22O11) are placed in a tall, 300 potassium chlorate (KClO3) with sugar and ignited with a 4 mL beaker. 70 mL of concentrated (18 M) H2SO4 is added. standard fuse. G. W. Eastland had previously used thermite to After several seconds, the reaction proceeds, producing a ff 5 ignite such a KClO3 mixture for e ect. Indeed, the KClO3 column of elemental carbon. ff reaction itself has been used to great e ect in its own right as a Decomposition of KClO tool for demonstrating reaction dynamics.6 3 A small quantity of KClO is placed in a mortar and exposed to We have found that the KClO3 + sugar mixture, chemically 3 ffi the flame from a small torch. Decomposition occurs, but the ignited with sulfuric acid (H2SO4), provides su cient energy to ignite a mixture of thermite. This also presents a valuable reaction is otherwise unremarkable. The interested student opportunity to perform several demonstrations leading up to could collect the evolved O2 and test for it. the thermite reaction which build upon simple chemical principles students have been studying. Prior to the final

H2SO4 + KClO3

A small quantity of KClO3 is placed in a mortar. To this, a few drops of concentrated (18 M) H2SO4 are added. Sugar is then sprinkled into the resulting liquid, producing sparks and sizzles with each crystal, which quickly subside. A tightly twisted paper towel may also be dipped in the liquid, which will rapidly combust.

H2SO4 + KClO3 + Sugar

KClO3 and sugar are mixed in a mortar in a 100:15 molar ratio mixture. To this, a few drops of concentrated (18 M) H2SO4 are added, initially producing the sparks and sizzles seen previously. After a few seconds, intense, continuous combustion will occur. Thermite A thermite mixture is prepared by mixing aluminum powder and iron oxide (Fe2O3) in a 1:3 mixture by mass. Enough volume to ﬁll 3/4 of a small garden planter pot provides for an exciting demonstration. Pots with small drainage holes in the bottom are preferred as they easily direct the ﬂow of molten iron. A layer of aluminum foil can be used to contain the thermite sample before ignition (see Figure 1).

Figure 2. Finished reaction showing the preferred arrangement of a double-layered flower pot suspended above a larger pot filled with sand to catch the molten iron flow.

Figure 1. Thermite mixture in aluminum foil containment prepared inside a ﬂower pot. A small divot in the center is formed for containing the ignition powder mixture.

A second pot layered with the first protects against the inner pot shattering during the reaction. Both pots should be suspended on a ring stand over a much larger pot filled with sand to collect and cool the molten iron flow (see Figure 2). Figure 3. Roman candle ignition mixture added to thermite divot. A second divot is made to accept the H2SO4. A divot is made in the thermite mixture, and the KClO3 + sugar mixture, prepared as before, is added. Shaping the mixture into a cone or steep mound produces the best results (see Figure 3). To this, several drops of concentrated (18 M) H SO 2 4 should be performed in a well-ventilated area: an outdoor are added. The combustion reaction is observed as before, followed quickly by the far more energetic thermite reaction, setting is preferable, as many of these reactions generate large which produces a flow of molten iron from the bottom of the quantities of smoke (some sulfurous), which can be agitating to pots which is caught in the sand below (see Figure 4). After a observers with respiratory conditions. Observers should be few moments, students can observe the red-hot molten iron. situated well back from the reactions, at least 15 or 20 feet from Once the glow recedes, the iron can be carefully retrieved with the estimated extent of the reaction. We strongly recommend a a pair of tongs and cooled in a water bath. A video of the trial run before any demonstration to determine an appropriate demonstration is available online.8 location and observation distance given the quantities of ■ HAZARDS thermite desired. A web camera could also be used to provide a For all demonstrations, proper personal protective equipment is detailed close-up of the demonstration for students to view. necessary including safety glasses, gloves, laboratory coats, long Hazards associated with specific demonstrations are listed pants, and close-toed shoes. The first four demonstrations below.

B DOI: 10.1021/ed500522c J. Chem. Educ. XXXX, XXX, XXX−XXX Journal of Chemical Education Communication ﬁ Decomposing KClO3 will produce signi cant oxygen (see eq 1), but the reaction terminates quite quickly as soon as the heat source is removed. heat 3 KClO (s)⎯→⎯⎯ KCl (s) + O(g) 3 2 2 (1)

As an alternative, H2SO4 can be used to facilitate the decomposition reaction indirectly. The reaction of KClO3 with H2SO4 creates chloric acid (HClO3) and potassium sulfate (K2SO4) via an exchange reaction (eq 2).

2KClO324 (s)+⇌ H SO (aq) 2HClO 3 (aq) + K 24 SO (aq) (2) While a relatively eﬀective method, this reaction terminates as soon as the limiting reagent, in this case H2SO4, runs out. Figure 4. Thermite reaction. The display shown here was prepared Adding the sugar to the mixture solves this problem by creating with roughly 10 times the amount of thermite as shown in Figures 1 a sustaining reaction mixture. The HClO3 initially reacts and 3 violently with the sucrose (eq 3), providing ample heat to decompose the remaining, unreacted KClO3 (eq 4).

8HClO3122211 (s)+ C H O (s) H2SO4 + Sugar →++11H22 O (g) 12CO (g) 8HCl (aq) (3) Concentrated H2SO4 is extremely caustic, and saturates the carbon column produced in the dehydration of sugar reaction. 2KClO32 (l)→+ 2KCl (s) 3O (g) (4) This reaction also produces very hot steam and SO2 vapor. H SO + KClO + Sugar The resulting oxygen excess, combined with the heat of the 2 4 3 decomposition, ignites the sucrose combustion, which con- The reaction proceeds quickly upon the addition of the H2SO4 tinues until the sucrose or the KClO3 is depleted (eq 5). The and emits sparks which may jump a considerable distance from addition of metal salts produces a colored flame, which provides the crucible. A quick retreat to a safe distance after the addition for a further discussion of atomic transitions. of the acid is recommended. Thermite C12 H 22 O 11 (s)+→ 12O 2 (g) 12CO 2 (g) + 11H 2 O (g) (5) When using a fine-grain aluminum powder, extreme care should be taken, especially against inhalation. In addition to full The reactive mixture is then used to produce sufficient standard personal protective equipment, respiratory masks are energy to overcome the activation energy for the thermite strongly recommended when mixing the thermite as the redox reaction (eq 6), thus achieving the goal (and the aluminum powder is difficult to contain. An empty parking lot, spectacular finish). large, wet lawn, or other suitably nonflammable, outdoor Fe O (s)+→+ 2Al (s) 2Fe (l) Al O (s) (6) location is recommended for this demonstration. For a 23 23 demonstration of the scale shown here (a small flower pot), Finally, this demonstration can also serve as an opening to students should stand well-clear (at least 15−20 feet) of the discuss kinetic versus thermodynamic control of a reaction with reaction vesselfarther if a larger quantity of thermite is used. students. While the thermite reaction is massively exothermic, If windy conditions prevail, students should stand upwind from its high activation energy means that the components can be the reaction, to avoid inhalation of any of the resulting fumes. safely mixed and stored until sufficient energy is input into the The reaction is extremely violent, with sparks of molten iron system via an ignition source, as demonstrated here. Although being thrown a considerable distance from the flower pot. A beyond the scope of this paper, the interested instructor could clear blast shield, large enough to reach a foot or more over the engage students by calculating the thermodynamics of the top of the flower pot and placed to shield the audience, can be individual reactions in this sequence. used for larger demonstrations or in more confined locations. After the acid is added to the mixture, the demonstrator should ■ CONCLUSION retreat to a safe distance to observe. If ignition fails, we By introducing a series of basic reactions one by one and recommend waiting a few minutes before attempting a second subsequently demonstrating how they can be applied together ignition. Once the reaction has subsided and sparks have in the ignition of the thermite reaction, students are shown how ceased, the reaction vessel should be safe for the demonstrator simple concepts such as combustion, dehydration, exchange, to approach with caution. and decomposition can be used in the context of overcoming large reaction activation energies. ■ DISCUSSION The thermite reaction possesses a high reaction energy ■ AUTHOR INFORMATION  barrier a simple combustion reaction with atmospheric O2 Corresponding Author will not produce enough heat. In order to provide the required *E-mail: [email protected]. energy, a combustion reaction requires a larger supply of O2. That O can be generated through the decomposition of Author Contributions 2 § KClO3. P.B.C. and A.N.B. contributed equally to this work;

C DOI: 10.1021/ed500522c J. Chem. Educ. XXXX, XXX, XXX−XXX Journal of Chemical Education Communication

Notes The authors declare no competing ﬁnancial interest. ■ ACKNOWLEDGMENTS B.A.M. gratefully acknowledges funding from a National Science Foundation Graduate Research Fellowship. At the time of publication, B.A.M. was a Jansky Fellow at the National Radio Astronomy Observatory. ■ REFERENCES (1) Nelson, J. E. The displacement series as a means of effective chemistry teaching. J. Chem. Educ. 1930, 7, 1895−1900. (2) Klein, O. C. An improved method for igniting thermite reactions. J. Chem. Educ. 1937, 14, 320. (3) Brockett, C. P. Thermite ignition assured. J. Chem. Educ. 1952, 29, 525. (4) Fortman, J. J.; Schreier, J. The use of fuses in chemical demonstrations. J. Chem. Educ. 1993, 70, 328−329. (5) Eastland, G. W. The thermite reaction: A chemical ground breaking. J. Chem. Educ. 1984, 61, 723. (6) Eliason, R.; Lee, E. J.; Wakefield, D.; Bergren, A. Improvement of sugar-chlorate rocket demonstration. J. Chem. Educ. 2000, 77, 1581− 1583. (7) Shakhashiri, B. Z. Chemical demonstrations: a handbook for teachers of chemistry; University of Wisconsin Press: 1983; Vol. 1,pp 77−78. (8) Thermite Demonstration Ignited by Potassium Chlorate “Rocket” Reaction. http://youtu.be/2dtrYtW8blM (accessed Nov 2014).

D DOI: 10.1021/ed500522c J. Chem. Educ. XXXX, XXX, XXX−XXX